Sustainable yield is the difference between annual median flow and the estimated environmental flow. The sustainable yield of a groundwater or surface water system refers to the volume and/or rate of water that can be extracted from that system while maintaining an acceptable level of aquatic ecosystem health. The National Water Initiative defines sustainable yield as the level of water extraction from a particular system that, if exceeded, would compromise key environmental assets, or ecosystem functions and the productive base of the resource.

The sustainable yield can indicate environmental stress on an aquatic ecosystem if water extraction is greater than the sustainable yield. It may also be useful to highlight those catchments where there is scope to increase water allocation.

The data for this indicator have been provided by DPIPWE (Internal link2006). Sustainable yield estimates are not available for catchments controlled by Hydro Tasmania. This applies to 8 of the 48 (17%) planning and management catchments in Tasmania, namely: Ouse, Upper Derwent, Lower Derwent, Gordon – Franklin, King – Henty, Pieman, Great Lake, and Brumbys Lake. Sustainable yield estimates have not been calculated for an additional 7 catchments, namely: Furneaux, Derwent Estuary – Bruny, Port Davey, Wanderer – Giblin, Nelson Bay, King Island, and Tamar Estuary. Two of these catchments, Port Davey and Wanderer – Giblin, are in the southwest WHA.

As noted in the description of this indicator, sustainable yield in Tasmania is defined as the difference between annual median flow and the estimated environmental flow. Work to improve the understanding of environmental flows is continuing in Tasmania and this will influence sustainable yield estimates such as those detailed in this indicator. The Water Resources Division of DPIPWE has developed a theoretical framework to assess environmental water requirements of ecosystems. More information on this framework is available from the External linkThe Tasmanian Environmental Flows Project Framework.

Sustainable yields need to be reviewed, for example, if step changes in streamflow occur as a consequence of prolonged drought (Internal linkStreamflow).

The data and analysis is simplified both in relation to time and scale. In particular, it does not identify seasonal variations such as allocations in summer time when it is more likely that rivers will experience stress. On a localised or sub-catchment scale many additional rivers may experience over-allocation.

A further caveat is that sustainable yield estimates are based on median average annual water yields and as such represent the average amount of water expected in the catchment 1 year in every 2. This amount of water is unlikely to be available annually in Tasmania. Accordingly, over-allocation of the sustainable yield is likely to be under reported in some catchments in this indicator.

The data in this indicator also pre-dated significant recent declines in rainfall over the reporting period for this SoE from 2003–08. Different analysis tools and criteria can produce significantly different results. For example, based on an 80% reliability criterion and better accounting for the more recent drought conditions during the reporting period, the sustainable yield for the Macquarie catchment was estimated to be 17,000 Ml/yr (unpublished DPIPWE data). This is in contrast with the sustainable yield for the Macquarie catchment of 300,665 ML/yr referred to in the indicator.

The disparity between the estimates also needs to be understood from the point of view that small changes in rainfall—and the reliability of that rainfall—can have significant impacts on streamflow and sustainable yield. A key conclusion of a 2002 paper is that changes in rainfall are amplified in runoff. In temperate catchments the percentage change in runoff is about twice the percentage change in rainfall. In ephemeral catchments with low runoff coefficients the percentage change in runoff can be more than four times the percentage change in rainfall (Internal linkChiew and McMahon 2002). Therefore, a key limitation with sustainable yield is how to define it or measure it in the context of climate change with the potential for system changes in water availability. The Climate Futures Program in Tasmania provides a key tool in understanding and anticipating these system changes in yield.

This information is also shown in the following maps of sustainable yield for Tasmania's planning and management catchments. The thematic map (below left) shows sustainable yield. The interactive map (below right) displays sustainable yields where available for each catchment (this map requires External linkAdobe SVG Viewer and is only available for selected web browsers).

Sustainable yield and licenced abstraction for planning and management catchments

The following table compares sustainable yield with licenced abstraction for each of the 48 planning and management catchments. A number of caveats and limitations with this analysis are detailed above.

As shown in the table, the Emu and Clyde rivers were substantially over-allocated against sustainable yield during the reporting period from 2003–08. Further, sustainable yield estimates are based on median average annual water yields and as such represent the average amount of water expected in the catchment 1 year in every 2. Because this amount of water is unlikely to be available annually in Tasmania, over-allocation may also occur in other catchments.

Sustainable water yield and licenced abstraction by catchment

Catchment

Sustainable yield (ML/yr)

Licenced (ML/yr)

% Allocated

1. Furneaux

(b)

75

–

2. Musselroe - Ansons

24,958

3,125

12.5

3. George

35,495

790

2.2

4. Scamander-Douglas

11,939

590

4.9

5. Swan - Apsley

35,620

1,169

3.3

6. Little Swanport

103,241

30

0.0

7. Prosser

137,326

465

0.3

8. Tasman

50,177

110

0.2

9. Pittwater - Coal

41,170

124

0.3

10. Jordan

23,545

511

2.2

11. Clyde

8,443

11,021

130.5

12. Ouse

(a)

3,423

–

13. Upper Derwent

(a)

74,241

–

14. Lower Derwent

(a)

175,582

–

15. Derwent Estuary - Bruny

(b)

–

–

16. Huon

640,854

46,559

7.3

17. Port Davey

(b)

–

–

18. Wanderer - Giblin

(b)

–

–

19. Gordon - Franklin

(a)

2

–

20. King - Henty

(a)

4,573

–

21. Pieman

(a)

8,986

–

22. Nelson Bay

(b)

–

–

23. Arthur

282,899

682

0.2

24. Welcome

5,292

146

2.8

25. King Island

(b)

17

–

26. Montagu

27,431

126

0.5

27. Duck

33,804

2,750

8.1

28. Black - Detention

36,035

1,080

3.0

29. Inglis

23,472

6,851

29.2

30. Cam

12,680

2,063

16.3

31. Emu

6,192

51,048

824.4

32. Blythe

8,896

1,274

14.3

33. Leven

58,298

5,746

9.9

34. Forth - Wilmot

107,270

9,871

9.2

35. Mersey

241,681

9,210

3.8

36. Rubicon

39,358

860

2.2

37. Meander

589,221

8,438

1.4

38. Great Lake

(a)

–

–

39. Brumbys - Lake

(a)

189,243

–

40. Macquarie

300,665

20,163

6.7

41. South Esk

817,410

30,521

3.7

42. North Esk

282,826

34,817

12.3

43. Tamar Estuary

(b)

2,139

–

44. Pipers

231,193

127

0.1

45. Little Forester

84,717

500

0.6

46. Great Forester - Brid

245,417

36,803

15.0

47. Boobyalla - Tomahawk

101,115

1,470

1.5

48. Ringarooma

347,395

6,860

2.0

(a) Sustainable yield data is not available for Hydro Tasmania catchments.

An indicator can show trends or changes that apply to one or more environmental issues. The data within an indicator is used to inform an issue report and any related recommendations. A summary of the indicator's relevance to a particular issue can be found within the 'Indicator' section of each of the linked issue reports below.